Recently, in view of a reduction of an environment load, an environment friendly and clean liquid fuel having a low sulfur content and a low aromatic hydrocarbon content has been desired. In view of this, as a technique for producing a fuel-oil base free of a sulfur content or aromatic hydrocarbons and rich in an aliphatic hydrocarbon, in particular, a kerosene/light-oil base, a method of using the Fischer-Tropsch synthesis reaction (hereinafter, sometimes referred to as “FT synthesis reaction”) employing carbon monoxide and hydrogen as raw materials has been studied (see, for example, Patent Literature 1).
A synthetic oil (crude oil) obtained by the FT synthesis reaction (hereinafter, sometimes referred to as “FT synthetic oil”) is a mixture containing aliphatic hydrocarbons having a wide carbon-atom distribution as main components. From the FT synthetic oil, it is possible to obtain a naphtha fraction rich in a component having a boiling point lower than about 150° C., a middle fraction rich in a component having a boiling point from about 150° C. to about 360° C. and a wax fraction (hereinafter, sometimes referred to as “FT wax fraction”) containing a heavier hydrocarbon component than the middle fraction (having a boiling point exceeding about 360° C.). Of these fractions, the middle fraction, which is the most useful fraction corresponding to a kerosene/light-oil base, is desirably obtained in a high yield. Therefore, in an upgrading step for obtaining a fuel-oil base from the FT synthetic oil, the yield of the middle fraction is increased as a whole by converting the FT wax fraction, which is produced together with the middle fraction in an FT synthesis reaction step in a considerable amount, into a component corresponding to a middle fraction by reducing the molecular weight by hydrocracking.
The FT wax fraction obtained from the FT synthetic oil by fractionation is hydrocracked in a wax fraction hydrocracking step and then separated into a gas and a liquid in a gas liquid separation step. The liquid component (hydrocarbon oil) obtained herein is fed to a rectifying tower downstream, together with a middle fraction, which is previously fractionated from the FT synthetic oil and separately hydrorefined, and fractionated therein to obtain a middle fraction (kerosene/light oil fraction). At this time, from the bottom of the rectifying tower, a heavy component (bottom oil), which mainly contains a so-called undecomposed wax which has not been hydrocracked in such a level to have a boiling point in the range for a middle-fraction, in a wax fraction hydrocracking step, is recovered. The whole amount of bottom oil is recycled by resupplying it together with a wax fraction from an FT synthesis reaction step to a wax fraction hydrocracking step and hydrocracking it again (see, for example, Patent Literature 2).
A technique for producing a hydrocarbon oil such as the aforementioned liquid fuel is known as a GTL (gas-to-liquid) process, in which a synthetic gas containing carbon monoxide gas and hydrogen gas as major components is produced from a gaseous hydrocarbon such as a naturally occurring gas used as a raw material in accordance with a reforming reaction and then the hydrocarbon oil is produced by using the synthetic gas as a raw material for the FT synthesis reaction.